cmd/internal/gc: separate Name-only Node fields

cmd/internal/gc: separate Name-only Node fields

Name will be converted from an anonymous to a
named field in a subsequent, automated CL.

No functional changes. Passes toolstash -cmp.

This reduces the size of gc.Node from 424 to 400 bytes.
This in turn reduces the permanent (pprof -inuse_space)
memory usage while compiling the test/rotate?.go tests:

test	old(MB)	new(MB)	change
rotate0	379.49	367.30	-3.21%
rotate1	373.42	361.59	-3.16%
rotate2	381.17	368.77	-3.25%
rotate3	374.30	362.48	-3.15%

Updates #9933.

Change-Id: I21479527c136add4f1efb9342774e3be3e276e83
Reviewed-on: https://go-review.googlesource.com/10120
Reviewed-by: Russ Cox <rsc@golang.org>